NONCOHERENT LDPC-CODED PHYSICAL-LAYER NETWORK CODING USING MULTITONE FSK

Abstract

A noncoherent two-way relaying system is developed using physical-layer network coding for improved throughput over conventional relaying in a fading channel. Energyefficient noncoherent operation is achieved using multitone frequency shift keying (FSK). A novel soft-output demodulator is developed for the relay, and corresponding achievable exchange rates are found for Rayleigh fading and AWGN channels. Biterror rate performance approaching the achievable rate is realized using a capacity-approaching channel code and a receiver architecture that iterates between demodulation and channel decoding. Iterative decoding is performed feeding information back from the channel decoder to the demodulator. Additionally, error-rate performance is made to approach the achievable rate more closely by optimizing LDPC codes for this system. The energy efficiency improvement obtained by increasing the modulation order is more dramatic for the proposed physicallayer network coding scheme than it is for a conventional pointto- point system. Using optimized LDPC codes, the bit-error rate performance is improved by as much as 1.1 dB over a widely known standardized LDPC code, and comes to within 0.7 dB of the limit corresponding to the achievable rate. Throughout this work, performance for physical-layer network coding is compared to conventional network coding. When noncoherent FSK is used, physical-layer network coding enables higher achievable rates, and conventional network coding exhibits better energy efficiency at low rates.

Existing System

Physical layer network coding (PNC) and differential phase shift keying (DPSK)

Proposed System

In this paper we developed a noncoherent modulation and coding system for DNC using multitone FSK. A novel soft-output demodulator was developed for the relay, and the achievable exchange rate was quantified. The relay receive architecture iterates between the demodulator and LDPC channel decoder to achieve bit-error rate performance that approaches the achievable rate. DNC was compared against link-layer network coding (LNC), a protocol where the terminals transmit to the relay using separate channel resources with no interference. The achievable rate analysis revealed that there is a threshold rate above which DNC exhibits better energy efficiency than LNC, and below which LNC efficiency is best. For DNC, increasing the modulation order from M = 2 to M = 4 yields as much as 3 dB energy efficiency gain, demonstrating the utility of M-ary FSK. Additionally, higher-order FSK exhibits greater energy efficiency gain over binary FSK for non-coherent DNC than for the single-source, single destination point-to-point channel. A simulation campaign investigated the error rate performance at the relay for DNC and LNC. In particular, several modulation orders were simulated with and without fading amplitude knowledge and using LDPC channel coding. Optimized LDPC codes for DNC were generated by an EXIT curve-fitting process. Variable node degree distributions were discovered which closely match the EXIT characteristics of the variable nodes to the check nodes. The optimized codes outperform well-known standard codes by up to 1:1 dB, and perform within 0:7 dB of achievable rate.

Architecture